The present invention relates to a flame-resistant plaster board, which is markedly inhibited in generation of smoke on heating or fire without reduction of the mechanical strength.
A large amount of plaster board is now in use as a flame-resistant building material due to the excellent flame-resistant performance and low-price of plaster. The flame-resistant and mechanical performances of a plaster board depend largely upon the property of the board paper constituting the outside of the plaster board. When a plaster board has a thin plaster layer, such tendency is more increased.
The board paper will burn on heating or fire, but the firing of the board paper is inhibited by the dehydration of plastic dihydrate, thus causing a phenomenon called "smolder". Accordingly, in the smolder, the generation of smoke tends to become large because of the controlling of temperature rise of the board material. The weight of the board paper constituting the outside of the plaster board is limited, for instance, to about 300 g/m.sup.2 in the case of a plaster board of about 9 mm in thickness, for practical use. Even with such limitation, the plaster board shows a smoking coefficient (C.A.) of only around 60, which is the second class of fire-resistance ("semi-non-inflammable") as specified by JIS (Japanese Industrial Standard) A-1321, or sometimes more. The term "smoking coefficient" herein used is intended to express an amount of smoke in the surface-heating test of a plaster board which is specified in JIS A-1321.
In order to maintain the excellent flame-resistant performance of a plaster board, non-inflammable mineral fibers such as glass wool and asbestos fibers may be used as a material for the board paper. However, these mineral fibers are relatively expensive and insufficient in adhesion to plaster. While it has been proposed to coat the surface of a plaster board prepared by a conventional method with a swellable fire-proof paint or to over the said surface with an aluminum foil, the plaster board thus obtained is not economical. Impregnation of the board paper as a material for a plaster board with an aqueous solution of a flame retardant such as ammonium sulfamate guanidinocarbonic acid, ammonium phosphate or ammonium polyphosphate can not produce a satisfactory smoke-inhibiting effect.
As an result of the extensive study on the economical treatment of a plaster board for enhancement of its flame-resistant performance, particularly of its performance in inhibiting the generation of smoke on heating or fire without reduction of the mechanical strength, it has been found that a remarkable smoke-inhibiting effect can be obtained by allowing a board paper to contain at least one alkali compound which easily decomposes or melts on heating. A plaster board with such board paper having, for example, a thickness of about 9 mm passes the grade "non-inflammable" when tested according to the method as specified in JIS A-1321.
According to the present invention, there is provided a flame-resistant plaster board characterized in that the board paper constituting the outside of the plaster board contains at least one alkali compound selected from the group consisting of hydroxides and salts of alkali metals and alkaline earth metals, which easily decompose or melt on heating.
As the hydroxides and salts of alkali metals, there may be exemplified MOH, MHCO.sub.3, M.sub.2 CO.sub.3, MAlO.sub.2, M.sub.2 O(SiO.sub.2).sub.1-3, M.sub.2 SnO.sub.3, M.sub.2 MoO.sub.4, M.sub.2 WO.sub.4, RM, R'M.sub.2 and R'HM (wherein M is sodium or potassium, R is a C.sub.1 -C.sub.3 monobasic fatty acid residue and R' is a C.sub.2 -C.sub.4 dibasic fatty acid residue). As the hydroxides and salts of alkaline earth metals, there may be exemplified M'(OH).sub.2, M'CO.sub.3, R.sub.2 M' and R'M' (wherein M' is calcium, magnesium or strontium and R and R' are each as defined above). Specific examples include sodium hydroxide, sodium carbonate, sodium hydrogencarbonate, sodium formate, sodium acetate, sodium oxalate, sodium succinate, sodium aluminate, water glass, potassium hydroxide, potassium carbonate, potassium hydrogencarbonate, potassium formate, potassium acetate, potassium oxalate, potassium succinate, calcium formate, calcium oxalate, etc. Economically preferred are salts of sodium and calcium. These compounds will be hereinafter referred to as "alkali compounds". The alkali compounds as mentioned above may be used alone or in combination.
The effective amount of the alkali compound for inhibiting the generation of smoke is about 1 to 30 % by weight, preferably about 3 to 15 % by weight, based on the weight of the board paper. When the amount of the alkali compound is less than the said lower limit, the smoke-inhibiting effect of the plaster board is much lowered. While the use of the alkali compound is an amount more than the said upper limit does not result in the elevation of the effect, it rather produces disadvantages such as exudation of the alkali compound to the surface of the board paper due to the hydroscopic property. The alkali compound is usually employed in the form of aqueous solution or slurry at a concentration of about 10 to 30 % by weight.
The flame-resistant plaster board of the invention may be prepared by treating a plaster board molded in advance with an aqeuous solution or slurry of the alkali compound to impregnate such solution or slurry into the board paper constituting the outside of the plaster board, or by molding a plaster board using a constituent of the outside a board paper previously treated with an aqueous solution or slurry of the alkali compound to impregnate such solution or slurry therein.
For instance, an aqueous solution or slurry of the alkali compound is applied to a plaster board so as to penetrate the solution into the board paper constituting its outside, optionally followed by treatment with a water-repellent, and the resulting plaster board is dried. This process is applicable irrespective of the kind of the alkali compound, and low-price alkali compounds such as water glass, sodium hydroxide and sodium carbonate may be employed.
Further, for instance, the alkali compound sparingly soluble in water (e.g. oxalates) in the form of aqueous slurry is applied to papers which are used to constitute the surface and inner layers of a board paper, and the resulting papers are laminated with a paper which is used to constitute the back side layer and is not applied the alkali compound to make a broad paper. Using such board paper, a plaster board which is flame-resistant may be molded.
Furthermore, for instance, an aqueous solution of the alkali compound showing nearly neutral such as formates or acetates is applied to a board paper, for instance, by dipping or coating; if necessary, the resulting board paper may be treated with a water-repellent, followed by drying. The thus prepared board paper is molded to make a plaster board. The use of such salt which hinders the adhesion between the plaster and the board paper as alkali metal salts other than formates and acetates is to be avoided.
In general, the board paper constituting or to constitute the outside of a plaster board is treated with a water-repellent, and therefore its treatment with the alkali compound does not sometimes assure the sufficient penetration unless a strongly basic alkali compound such as sodium hydroxide, water glass, sodium aluminate or sodium metasilicate is used. When the alkali compound does not sufficiently penetrate into the board paper, it may be crystallized out on the surface of the board paper on drying so that a satisfactory smoke-inhibiting effect is not exerted. Because of this reason, the pre-treatment of the board paper with a penetrating agent is recommended. As the penetrating agent, there may be used an aqueous solution of a surfactant, a water-soluble lower aliphatic alcohol, aqueous ammonia or the like. Among them, the use of a surfactant is particularly preferred. In case of using a surfactant as the penetrating agent, an aqueous solution containing about 0.5% by weight of the surfactant may be impregnated into the board paper in an amount of about 50 to 100 g/m.sup.2.
As the result of the treatment with the alkali compound, the board paper may tend to lose the sizability resulting in deterioration of water-resistance. Therefore, the plater board after the treatment with the alkali compound is desired to be treated with a water-repellent. As the water-repellent, there may be used, for instance, paraffin waxes, petroleum resins, polyfatty acids, silicon resins and other synthetic resins, preferably in the form of aqueous emulsion. For instance, their aqueous emulsion having an available solid content of about 5% by weight may be applied to the board paper treated with an aqueous solution or slurry of the alkali compound in an amount of about 30 to 50 g of the aqueous solution or slurry per m.sup.2, followed by drying. The resulting board paper will have a satisfactory water-repellency. In this case, however, care should be taken to avoid the attachment of the water-repellent to the board paper in a too large amount, since otherwise the generation of smoke will tend to increase on heating.
As mentioned above, the penetrating agent, the alkali compound as a smoke-inhibiting agent and the water-repellent may be applied to a plaster board or the board paper constituting its outside in separate operations. It is, however, more convenient to formulate these three agents in a single (i.e. one liquid type) composition and apply such composition to the plaster board or the board paper by a single operation.
The said one liquid type composition may be composed of the penetrating agent (e.g. aqueous ethanol), the alkali compound (e.g. sodium formate) and the water-repellent (e.g. petroleum resin, polyfatty acid) dispersed in an aqueous medium. For this purpose, a surfactant of the formula: ##STR1## wherein R.sup.1 is alkyl, aralkyl, A and B are each hydrogen or R.sup.1 O--(CH.sub.2 CH.sub.2 O).sub.n -- and n is a number of about 1 to 30 in the form of, for example, about 0.5 % by weight of aqueous solution is advantageously used as the penetrating agent.
The one liquid type composition may be prepared, for example, by firstly dissolving sodium formate in about 20 to 30% by weight aqueous ethanol so as to make a concentration of about 15 to 30% by weight of the sodium formate and mixing the resultant solution (about 90 to 85 parts by weight) with a petroleum resin emulsion having an available solid content of about 5 to 10% by weight (about 10 to 15 parts by weight).
The thus obtained one liquid type composition is stable and exhibits a remarkable smoke-inhibiting effect when applied to a molded plaster board, for instance, in a proportion of about 20 to 300 g/m.sup.2.
As for the smoke-inhibiting effect on a plaster board, the alkali metal salts are generally superior to that of the alkaline earth metal salts. On the other hand, the alkaline earth metal salts are usually superior to the alkali metal salts in the adhesion between the plaster and the board paper. It is favorable to treat a board paper with mixed salts (i.e. alkali metal salts and alkaline earth metal salts) which have the combined characteristics of the both metals and to mold a plaster board using the thus treated board paper. For example, a board paper may be dipped in or coated with an aqueous solution containing sodium formate and calcium formate in a weight ratio of 1:0.0 1-2, preferably 1 : 0.1-1. The penetration of the solution is promoted by previously applying the said penetrating agent to the board paper or by incorporating such penetrating agent into the solution. The resulting board paper may be, if necessary, treated with a water-repellent.
A plaster board can be molded by charging a plaster slurry, which is prepared by kneading water and plaster halfhydrate, between the two pieces of board paper and hardening the plaster slurry, followed by drying. The plaster halfhydrate may be in either of .alpha.-form or .beta.-form. Further, in order to improve the adhesion between the plaster layer and the board paper, organic adhesives such as polyvinyl alcohol, methyl cellulose, dextrin and synthetic resin emulsion may be incorporated into the plaster slurry.
The smoke-inhibiting treatment of this invention may be applied to a lightweight plaster board which contains air bubbles or lightweight aggregates in the plaster layer.